The invention relates to a method for operating an inverter for coupling an electric machine, designed for operation on AC voltage, to an AC voltage network with which a network-side converter of the inverter connected to the AC voltage network is electrically coupled by means of a DC intermediate circuit to a machine-side converter of the inverter connected to the electric machine, wherein conversion of electric energy by the network-side converter is controlled by means of a network control signal and conversion of electric energy by the machine-side converter is controlled by means of a machine control signal of the control unit. The invention also comprises a computer program product. Finally, the invention also comprises a control unit for operating an inverter for coupling an electric machine, designed for operation on AC voltage, to an AC voltage network to which end the inverter comprises a network-side converter connected to the AC voltage network and a machine-side converter connected to the electric machine, which, for the electric coupling, are connected to a DC intermediate circuit, wherein the control unit comprises a network-side connector for connection to the network-side converter and is designed to provide at the network side a network control signal to control the network side converter and wherein the control unit comprises a machine-side connector for connection to the machine-side converter and is designed to provide at the machine-side connector a machine control signal to control the machine-side converter.
Methods according to the invention for operating such inverters and control units for the control thereof are known in the prior art, for example from DE 10 2000 11079 905 A1, which discloses a method for operating a drive system. Inverters of the generic type are used to couple one or more electric machines, in particular rotating electric machines, to the AC voltage network, in particular a multiphase, preferably three-phase, AC voltage network. Such inverters are typically used with high powers, for example one or more MW or more. Such high powers give rise to special requirements because the operation of the electric machine with an inverter can result in interactions with the AC voltage network. Since the electric machine is as a rule designed for operation with AC voltage or AC current, it is necessary for the inverter to establish an energy-related coupling between the AC voltage or AC current on the network side and an AC voltage or AC current on the machine side. To this end, the network-side converter is provided, which is connected with its AC voltage side or AC current side to the AC voltage network and with its DC current side to the DC intermediate circuit. In addition, the inverter comprises the second machine-side inverter, which is connected on DC current side to the DC intermediate circuit and on the AC voltage side or AC current side to the electric machine. Hence, the two converters are electrically coupled to one another by the DC intermediate circuit so that energy can flow from the AC voltage network to the electric machine and preferably also vice versa.
A converter is an electric facility used for energy conversion. The converter establishes an electric coupling between an AC voltage or AC current on one side and a DC voltage or DC current on the other. To this end, as a rule, the converter comprises a plurality of semiconductor switches, which are suitably controlled on the basis of a control signal, namely the network signal or the machine signal, in order to convert the energy in the specified way. With the powers to be converted in the present case, as a rule, thyristors are provided as semiconductor switches. However, basically, the semiconductor switches can also be formed by transistors, for example bipolar transistors, field-effect transistors or the like.
If the inverter only comprises the machine-side and network-side converter, the electric energy supplied to the DC intermediate circuit by one of the converters must be dissipated again by the other one of the two converters. Hence, the converted power is to be adjusted by controlling the two converters by means of the control signals of the control unit assigned to them. The energy store for the DC intermediate circuit is provided by an electric coil with an inductor corresponding to the electric energy to be stored.
During use as prescribed, oscillating torques can occur on the electric machine, in particular interharmonic oscillating torques, which can be caused by the fact that a network side and machine side of the inverter cannot always be completely decoupled. As a rule, the amplitudes of the oscillating torques are comparatively low, frequently less than 2% of a rated torque. However, if mechanical natural frequencies of the electric machine are excited, for example natural frequencies, which are less than 50 Hz, this may give rise to much higher oscillating torques on the machine side, in particular high oscillating torques, which can, in particular, also result in critical operating states.
To avoid this problem, it can be provided that a rated frequency and a pole pair number of the electric machine for stationary operation can be selected such that critical excitation of natural frequencies can be avoided, for example using a Campbell diagram. This measure is obviously only suitable for electric machines operated in steady state under fixed predetermined operating conditions.
If, on the other hand, it is provided that the electric machine can assume different operating states, it can be provided that so-called skip frequency bands are provided which define frequency ranges, which, in operation as prescribed, cannot be actuated in steady state. Even though this method has proven itself, it has still been found to be disadvantageous in that it is inflexible with respect to loading of the electric machine. This is inter alia an obstacle with regard to process control because there is a possibility that required operating conditions cannot be operated in steady state. Namely, a change in a load or the network frequency can, for example, have the result that the defined frequency ranges no longer match the natural frequencies that are actually present and hence it is no longer possible to avoid the above-described problem of the excitation of oscillating torques.
A further possibility consists in the detection of real actual torques or actual torsional moments by suitable sensors and their attenuation by controlling suitably by means of the control unit. Such a procedure is known, for example, from WO 2006/113230 A1 and also from EP 2 073 375 A1. However, this measure has the drawback that effort is required to measure the mechanical properties on the electric machine, in particular in the case of varying combinations of inverters with electric machines, this can entail a high degree of effort not only with respect to structural requirements, but also with respect to the implementation of a suitable control system.
The invention is based on the object of providing an improved method for operating an inverter and a control unit for this purpose.